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Topic: Racer Resurrection (Read 40672 times)

Thought a quick introduction was in order as I start to resurrect this mighty beast of a race bike. I'v just acquired a new to me BS 175 that was previously raced with AHRMA. It's hasn't been on the track for a couple of years I think, having suffered a catastrophic crank failure - you can see in the pics below. The good news is it came with a cash of 2 spare cranks. One NOS, never run and the other in good form with the oil pump gear already removed.

Having never changes from when I was a kid. The first thing I did when I got it home was to take it apart and see what we had to work with. I knew the crank was bad when I bought it, and had assumed that some internal case damage and piston damage had occurred. Pulling the jugs relieved small ghosting in the chrome on the left barrel. That was easily honed out and the bearings moved from one crank to the other. Everything else in the bottom end looks exceptionally clean. I think I lucked out!

But I do have some general questions.

1. Can someone identify the modification to the rotary covers? They are larger than stock, and do not have the razed mount for the oil pump.

2. Has anyone ever seen these carb covers? The previous owner mentioned that he acquired some race parts and I assume this is one of them. How common were these, and are they factory or after market.

The rotary valve covers, carburetors and side covers are the factory SR175 racer parts. All are very rare, although there are probably a few pieces stashed in people's garages that occasionally show up on e-bay. The bike also has an SR alternator (aluminum center section, no windings for battery charging) and appears to have something other than the stock points ignition system since the alternator end cover is spaced out from the body.

The bike may have SR cylinders or standard DT cylinders, you'll have to measure the parts you have. Either bore could be used in AHRMA. Don't buy pistons and rings until you've determined which you need.

As to history, the last time it was raced was at Grattan. This is where it spun the con rod - causing the blue. Im not sure what year that was, but it was a couple of years ago. I cant seem to find the old results on the AHRMA website.

It was previously owned and raced by Dave Dunfey as a trial bike while he was putting together his Vincent. When it wrapped the rod I think he moved over to the Vincent. Cant blame him, beautiful bike.

I'll need to do a second measurement to be sure, but over the weekend we measured 1.970". So that looks like it indicates a DT bore and not an SR. I didn't get a set of pistons with the bike, or spares - I suspect they got lost over the years.

1.970" = 50.038mm1.965" = 49.911mm

And no Paul, the rods are not 180 from each other. Luckily I did get 2 cranks in the deal. So one future spare and one to rebuild.

If you're planning on putting it on the street as a cafe bike or ?, you would want to change the alternator. Of course you'll have to wire in a rectifier and a few other wiring additions. You should see what kind of ignition is in that alternator before going too far, you could probably transfer it to a stock one if it is a decent electronic ignition.

I'm planning on keeping it a race only bike. No streets in it's future. But you make a good point, I need to open it up and get a better idea of what the current ignition is before I start making to many plans.

I found Dave Dunfey in the AHRMA results from 2009 and 2011 in the Class C Foot Shift class, which is probably where a Vincent would run. I didn't see anything from the 200GP class or Formula 125, the classes the Bridgestone would normally run. Perhaps the crank let go before he got a finish on it. There doesn't seem to be archived results prior to 2009.

In looking at some of your other pics, the cylinder shows 177cc which should indicate the pistons are 50mm. The SR cylinders have that removed. Also looks like an early motor as there are no locating peg for the center bearing and the bearing has oil groove.

I think some discussions have been had on other threads about pistons, and not running stock pistons.

Vince, in the UK talks about running a KX100 piston at 52.5mm with a re-bore and lining. Then later about a Wiseco 50mm to 52mm from an RM100.

Anyone care to chime in on piston recommendations?

I've also been looking at Vince's post about using a Yamaha RD200 head to get the advantage of a squish band. But I'm not sure the legality in AHRMA or USCRA. Any thoughts?

My past experience with stock pistons is that the rings didn't live very long when revved to 11,000 regularly. A couple of us that race the 175's are using Wiseco Suzuki RM85 2nd oversize pistons. However, to use them correctly, you have to shave the top of the cylinder or run without a head gasket, plus reshape the head to keep a reasonable compression ratio. The Suzuki pistons have a .035 inch shorter pin-to-top distance and a flatter top. The Suzuki pistons require another .005 inch clearance to keep from seizing, so can't just be slapped into a stock cylinder.

I'm not certain that you need a Yamaha head for the squish if you use the Suzuki pistons. Here is a normal BS 175 SR head that provides about 13:1 CR with stock pistons. Below that is a Bridgestone head modified to maintain the same CR with the Suzuki pistons. Since the head has to be cut so much due to the flatter top of the Suzuki piston, squish is pretty easy to control when machining the head.

I run Wiseco Suzuki RM85 pistons that are 50mm in my 175 racer. AHRMA has limitations on the carb size (22mm) and a max overbore of about 1.5mm for gp200. Vince has different rules to allow 52.5mm and has a iron liner. I have the cylinders nikasil plated. These bikes have transmission and crank weaknesses. I took my bike to the dyno once and their dyno could not read the ignition (old ignition) so I don't have a hp figure.

I've worked on the trans the last few years and it is much improved. I still want to hard chrome the shift fork tips as I don't have any new ones. Since I broke another crank earlier this year, that's the first thing on the list to fix. Maybe after that, I'll try and get it back to the dyno.

Paul and I have broken numerous crankshafts and Craig Hirko has broken at least one. I'm not certain about others that have raced the 175. Vince has the answer on that; better center bearing support. The transmissions like to jump out of gear. I never fixed mine, just lived with it, Paul has made progress on the stock trans, Vince is using an expensive aftermarket special. Like racing any stock street bike, especially vintage ones, they all have their weak points.

Hello Al.To clarify a few points..i run the KX100 piston at 52.5 in ceramic coated cylinders that are far superior to nicasil items.As Paul says we do have different rules here and can run at full 200cc..in fact i can run at 54mm if i need to as thats 1mm oversize on the 200cc Bridgestone that is eligible here.The RM100 piston(50mm type Wiseco454) is around 1mm taller pin to crown and the peg needs moving.Dependent on the tune id just jack the cylinder up on a gasket.Theres much to be said for choosing which parts you are going to run and building an engine around those parts,as Karl says theres usually a bit of skimming etc involved.In my experience the Wisecos are a fit and forget proposition though keep an eye on the ring pegs bedding inwards with miles.The cranks can be made to last even without a fourth bearing..but a straight pin is a nescessity.I havent broke one yet.Transmissions are a pain..the reality is new gears need making with a decent undercut on them,something thats impossible with round dogs/gears.Karls head mod is ok though..as long as the end result is what you need how you get there is of no consequence.

Hi Paul. I run both cylinders..the 54mm are on liners and i have a pair of lined ones im setting up for the RM100 piston. Current engines with the KX100 52.50 piston are coated with Apticote.Highly recommended.I found that the cylinders would "bed in" when i had them nicasiled and leave a wear edge at the top of the cylinder.Almost as if the cylinder had withdrawn if you like.The Apticote cylinders are still showing the original hone marks after two years use.

The best set up (for me) has been the Apticote cylinders running kx100 pistons.I suspect that should i have a pair of 200 (Mach11)cylinders plated and run with a decent piston then power will go up again..ive managed to get much more transfer area using those.

I'm not sure of the actual RM85 oversize but I use the Wiseco 806M0500 which is a 50mm piston. I think the kits now come with ring (1969cs), cir-clips (cw14) and a wrist pin (s288). I use the Wiseco B1012 wrist pin bearing (top end bearing) which I order extras of. Like Vince said, you want to get everything lined up before you go ahead with the motor. I've had cylinders nikasil plated at Millennium Technologies. I think it was around $175 per cylinder last time I had them done. There is a place in Wisconsin that does the ceramic plating that I think Vince is talking about called Max Power RPMs. Looks like they charge around $225 a cylinder. I may give them a call after the holidays to find out more about the process.

I am the owner of a 175 that has seen much better days, I also have what my wife calls a sick obsession with older racing bikes. The 175 in question has the right piston seized but the crank is free, i have no carbs and the clutch seems to be toasted. I would like to build this with the intention of racing it someday, if or when that will happen is questionable (we all gotta have dreams). If using the rm85 pistons is it only the head that needs to be 'altered' or does the top of the cylinder need the same treatment. Also is the portion removed strictly the 1 mm raised gasket area on the head or is there some machining of the combustion chamber. I am in need of carbs as well and was curious if the mikuni vm18-144 would work. I don't have any original carbs to take measurements and wanted to know if those new mikuni carbs were even an option. Marcus

I machined the top of the cylinder .035" to compensate for the roughly .035" shorter distance from the pin to the crown on the Wiseco Suzuki piston vs. stock. I order to get the higher compression ratio (approx. 13:1) while using the stock head gasket, I machined the head to 7.2 cc volume (see photo in reply #10 in this thread). Unfortunately, I can't find my note as to how much that actually was in linear measurement, you'll have to measure with a syringe & plate. You can see the squish that I ended up with. That was a 15 Deg. cut.

Paul Piskor (bsracer) did his slightly differently to get similar results with the Wiseco pistons. He was running without head gaskets, I believe, and may not have cut the top of the cylinders.